The gene-enzyme system alpha-glycerophosphate dehydrogenase in Drosophila melanogaster is well suited for a study on the genetic control of enzyme expression during development since: (1) the enzyme exists as multiple molecular forms which differ in kinetic parameters; (2) all isozymes are apparently the product of a single structure gene; (3) the isozymes are under stringent regulation since they are specific for certain larval and adult tissues and are distinct in their developmental expression; (4) each isozyme appears to participate in a distinct metabolic pathway; and (5) the enzyme is not essential to survival and hence, mutations affecting its development or tissue specific expression may be expected to be viable. The proposed research will focus on (I) the purification and a structural and biochemical comparison of the two major isozymes, (II) The identification of the epigenetic event that differentiates each GPDH isozyme with respect to kinetic and physicochemical parameters; (III) The isolation of mutants affecting the temporal, tissue-specific, and high and low intensity expression of the enzyme. These regulatory mutants will be characterized according to their position relative to the structural gene and to each other, their mode of operation, their effect on the structural gene product (i.e., are the enzymes molecules qualitatively altered in any way compared to a control class) and whether these mutants represent alterations in the rate of synthesis or degradation or the GPDH molecules.